question about a hissing noise from engine - Kawasaki Motorcycle Forums
 
LinkBack Thread Tools
post #1 of 7 (permalink) Old 04-12-2016, 02:54 PM Thread Starter
Member
BTK Beginner
 
Join Date: Jan 2016
Posts: 49
question about a hissing noise from engine

1990 Ninja 750R. Not the normal hiss from the carbs (CVK38D )but a much louder , somewhat intermittant hiss. Most noticeable when decelerating.

Previously the stock air box was replaced with pod filters. Dynojet kit installed. Eveporative/emmisions pieces removed.

I took the carbs apart and cleaned everything. Replaced the reed valves in the valve cover with block-off plates.

Also maybe related - the engine has a big flat spot at about 3/4 to full throttle. Does not seem to go away when the "choke" is turned on to supplement the fuel mixture.

Clears up and runs up to full RPM after several seconds of stumbling. Happens mostly at low RPMs. Air pistons start bouncing up and down with the bike stationary, pod filter removed and the throttles are suddenly cracked wide open.

hiss is : Crankcase breather maybe ? Has anyone come across this before ?

flat spot is : somebody drilled too large a hole in the air pistons when dynojetting the carbs. Does anybody know what the stock hole size is ?
tyrebyter is offline  
Sponsored Links
Advertisement
 
post #2 of 7 (permalink) Old 04-12-2016, 04:07 PM
Vintage Motor Mechanic!
BTK Expert
 
Join Date: Oct 2004
Posts: 1,054
Your post has me a bit confused... you are opening the throttle all the way and the bike bogs? Is that the case? Normally that would be the pilot air jet is too large and should be swapped for a smaller one. Not sure if that is possible with your carbs. The Dynojet kit isn't well thought out... suspect too rich in upper end of rpm range. I would try some stock jet needles as they are better matched to the needle jets / emulsion tubes and try a 135 Keihin main jet and one size up on the pilot jets; perhaps 40s? Drilling the slide is a waste of time as it is supposed to allow for better throttle response and is an old Harley CV carb trick that doesn't really do anything. Doesn't much matter how large the hole is; it is just there for relief so the spring can return the slide without any air pressure restriction.

I would guess that your whistling was the crank case breather. If you have one of those little filters stuck on it, just take it off and if it is open, stick a long hose on it and run the hose over the swing arm and see if that clears the noise up. There isn't much on an engine that can make hissing noises other than the crankcase vent I can think of...

wiredgeorge
Texas Hill Country
wiredgeorge is offline  
post #3 of 7 (permalink) Old 04-13-2016, 10:18 AM Thread Starter
Member
BTK Beginner
 
Join Date: Jan 2016
Posts: 49
Wiredgeorge, thanks for the reply.

You may remember me from my posting last Fall/Winter regarding this bike.

I will try to be less confusing.

It doesn't seem to matter what gear. Small throttle opening, everything is ok at all RPMs.

Slowly accelerate in 1st or 2nd gear, to 7,000 RPM and there is no missing or bog. It's smooth all the way up. Have not tried that in higher gears yet. Have not tired to take it to any higher RPM yet.

Problem is with about 3/4 (more or less) throttle to wide open throttle. More than just a bog. It misses terribly, spuuters loudly and actually looses RPM. Worse at low RPM. If I do it at higher RPM it stumbles/misses for a few seconds without slowing down and then suddenly it clears up and goes like a bat-outta-****.

It's a little scary because you don't know when it will clear up. There is no warning. Suddenly it just takes off. If I hold full throttle long enough, it will always clear up eventually, regardless what RPM. Took a while ... several test drives to work up enough courage where I could hold it at WOT long enough to verify this.

I believe if it was too lean, it would not clear up no matter how long I held it at WOT. Adding "choke" seems to yield more evidence that it's not too lean, because it does not help at all. Have not been able yet to determine if it is making things worse, however. still need to get the bike inspected. Waiting for tires.

I hear what you are saying about the Dynojet kit and especially the air piston vent hole. Carbs have : unknown main Jet # 132, Keihin pilot Jet # 35. No marking on the needle. I included a photograph if it helps you tell me it's Dynojet or not. It's dirty, yes, I know that.

I'm not sure it installs properly. It travels up and down in the slide by about 1/8". I'm considering putting small springs in under the plastic retainer to bias them down against the slide, yet retain the free movement for self-centering. Any other suggestions ? Do the stock needles fit with less up/down play ?

So according to your experience I am looking at getting stock replacement needles and mains around 135 and pilots around the 40 size.

You mentioned the stock needles should be N14Q. The workshop manual calls for N53I. Also it calls for 112 Main or 118 in California, which I am not. The VIN does not make it to be a CA model either.

I agree, it's worth trying the no-name needles for now and just changing jets. It's just that going from 112 to 135 mains seems like a very big jump, to me. Do you ever "rent" or loan out, a collection of jets ? Too bad my Mikuni jets won't work. I have a bunch of them right in the ballpark of what I'd like to try. Left over from setting up my 850 GS.

I'm planning to remove the breather cover and have a look to see what I can see. In the parts diagram it shows an "orifice" looks like a jet to me for regulating airflow. That piece might be the source of the hiss/whistling. Actually whistling is probably a better description of the sound. Apparently, this bike has always made this sound.

I understand your suggestion about running a length of hose between the breather and filter. Will probably put one on regardless of the noise. Have read that it can also help to prevent the normal amounts of blowby, from expelling oil droplets.
Attached Images
File Type: jpg noname needle.jpg (304.9 KB, 4 views)
tyrebyter is offline  
 
post #4 of 7 (permalink) Old 04-13-2016, 06:20 PM
Vintage Motor Mechanic!
BTK Expert
 
Join Date: Oct 2004
Posts: 1,054
Well, that is a DJ or other aftermarket jet needle; they all look the same normally with 6 slots rather than 5 or just one. I made some suggestions and don't have much to add. I don't "rent" anything. Sorry. And why bother with a filter on the crankcase breather. Take everything off an run it open and see if the whistling stops. I don't like filters at this point as they don't filter anything; just clog any oils drops coming out the vent... it is a vent. If you are concerned about blowby getting on thigs, that is where the hose comes in.

wiredgeorge
Texas Hill Country
wiredgeorge is offline  
post #5 of 7 (permalink) Old 04-14-2016, 11:24 AM Thread Starter
Member
BTK Beginner
 
Join Date: Jan 2016
Posts: 49
OK thanks wiredgeorge,

I see what you mean by a filter is probably not needed on the breather if a long enough tube is added. There was a whole bunch of greasy buildup directly underneath the filter, on the case. I guess I was more worried about stuff getting in than I was about oil vapors clogging a filter.

Just thought I'd ask about the jet "rental" possibility. Hey, I can dream can't I ?

Thanks for your help. I hope other people realize how much time you can save them, as you are with me. I'll update this thread in the next week or two, once I see the bike again and deal with the whistling noise.
tyrebyter is offline  
post #6 of 7 (permalink) Old 04-15-2016, 10:37 AM
Vintage Motor Mechanic!
BTK Expert
 
Join Date: Oct 2004
Posts: 1,054
The only thing that can get into the breather for the most part is mud daubers and other such bugs. Those things leave a solid mud clump in hoses and if outside, in power sockets and such. Not sure where you are from but down south they are a fact of life. They love to clump hoses and if a hose gets clumped, pressure can build in the crankcase and engine seals will blow out as a result. I am sure the good Lord had a purpose for these critters but for the life of me, I can't figure any reason for them to be on our planet sharing hoses with my bikes!

wiredgeorge
Texas Hill Country
wiredgeorge is offline  
post #7 of 7 (permalink) Old 05-17-2016, 08:08 AM Thread Starter
Member
BTK Beginner
 
Join Date: Jan 2016
Posts: 49
Wired George. You hit another home run. I took off the breather cover and drilled out the hole from about 1/8" to about 7/16" Whistling is GONE. Thanks again.

There was no separate piece or orifice as shown on the parts diagrams. Just a little rubber piece to prevent the windage tray above the breather internal filter/screenscreen, from flapping.

I put the K&N breather external filter back on and in the same place. Directly attached to the breather cover. If it gets wet with oil/condensation and begins to drip, I'll relocate it at the end of a long piece of tubing, per your other suggestion.
tyrebyter is offline  
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the Kawasaki Motorcycle Forums forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid email address for yourself.

Email Address:
OR

Log-in









Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.



Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome